Many medical procedures require the size of a body structure in a patient to be determined. When a treatment device to be used internally relies for optimal functioning upon correct sizing to the body structure, measurement inaccuracies risk compromising treatment. Examples include embolization procedures where a vessel needs to be sized before delivering an occlusion device or the like, and procedures for the placement a stent or graft. Typical vessel sizing prior to performing a procedure includes first calibrating an imaging machine to the known size of a catheter tip, or a known distance between points on a sizing wire. A technician will then typically place two points on a screen of the imaging machine, one on each side of the vessel across the diameter of interest. The imaging machine will then compare the calibrated measurement to the current distance between the two points across the vessel to determine vessel diameter. A treating physician can then select a device to be placed within the vessel based upon the determined size.
The above general technique is of course subject to human error. For instance, if the two points placed on the screen are not placed exactly perpendicular to the width of the vessel, an inaccurate measurement may be taken. Another issue with standard techniques is that the imaging machine must be perfectly in plane, or nearly so, with the plane of the vessel at the measurement location. This can be very difficult to establish. If the imaging machine is not properly oriented, calibration may be inaccurate, and as a result so can the measurements taken. It also commonly takes ten minutes or even more to properly position and calibrate the machine, and measure the target vessel.
U.S. Pat. No. 7,856,730 B2 to Sakai et al. proposes an internal diameter measurement device that would appear to take some of the potential for human error out of vessel measurement. In Sakai et al., a tubular sheath is inserted into a body cavity, and a balloon provided on the distal end of the sheath is inflated to displace a linear reference member providing an indication of the internal diameter of the cavity. While the device of Sakai et al. may function adequately, it is not without shortcomings.